SP170 dies @ full throttle unless choked

F

FamilyMechanic

Joined
May 28, 2024
Messages
5
Subaru – Robin SP170 engine installed in a 27 ton log splitter. It was not winterized last year and the carb was beyond recovery. Lots of rust and corrosion to the point the jets could not be removed. We threw it in the garbage and ordered a new one. After installing the new carb and it ran great for a couple tanks and then died. It still starts and idles fine but much over ½ throttle and it will die. After trying various things, I decided maybe the new carb could have failed so I ordered another new one (#2). Sadly, it had the exact same problem. In my quest to fix it thus far, I have:

Replaced the spark plug (has good spark)
Replaced the carb/intake gaskets
Sanded the spacer so it was flat on both sides
Checked coil spacing (within spec)
Oil is full (no oil safety switch on engine)
On/Off switch is bypassed
Cleaned gas tank (spotless now)
Replaced fuel line
Fresh Gas
Valve lash set to spec

What is frustrating is that it will run at full throttle if the choke is on. This leads me to believe there is air getting in somewhere.. but where? I have sprayed starting fluid and carb cleaner everywhere and nothing changes the engines behavior unless I spray it down the throat of the carb. The only things I have not messed with is to replace the coil or play with the governor. I have disconnected the governor and adjusted the throttle manually and it exhibits the same issue as if it was properly connected. I am at a loss and am usually pretty good at troubleshooting – especially SIMPLE small engines but this one is kicking my butt!! Any suggestions would be much appreciated ☹
 
Sounds like the engine is being starved for fuel. Have to figure out why. The fact that it will idle fine would seem to indicate there is no air leak in the system and it is just running out of gas at higher load. A lot of small engines without a fuel filter will have a strainer built in to the connector on the bottom of the gas tank. Check to see if you are getting a full stream of fuel flowing out of the tank.

I've never seen one of these engines so I don't know if this applies, but some carbs will have a small bowl underneath the fuel shutoff lever, this bowl has a built in strainer in it also.
 
Last edited:
Does it run full throttle on a steady stream of brake cleaner? Start there. Like Gyro said above, the bottom of your tank appears to have a threaded tank union that might be clogged. An easy way to test that without taking the tank off and getting gas everywhere is to pinch the existing fuel line and replace it on the carb with a test gas line full of gas on a gravity feed.
 
Last edited:
All good ideas and I will try them tomorrow when I get home from work.

To answer some of the suggestions - if I pull the bowl, gas streams out at a good pace. It does have a small bowl as suggested at the bottom of the fuel shutoff valve too. If I remove the at bowl, it provides a clean - steady flow as well. This seems to rule out the strainer/fuel line and bowls being clogged. Also - same issue on 2 different but identical carbs.

I like the idea of the float being off but not sure how I'd go about changing it. The float is plastic and not very adjustable...
 
The first thing to do is drain the float bowl. As mentioned it has some moisture/water.
 
This sounds like my old snapper lawn mower. I cleaned the carb and replaced seals and whatnot, then re-installed it with a new fuel line. It would idle ok for a while at least, but wouldn’t run under load. I found there was fuel in the bowl, but not enough. Finally I got it flowing through the needle and it worked fine since. I think the issue actually was some kind of air lock from the tank to fuel valve to carb. Letting it run a bit with the bowl off, and cycling the fuel shutoff fixed it.

It seems like it was actually the sequence that I connected up the fuel line, and purged air.
 
wpod said:
The first thing to do is drain the float bowl. As mentioned it has some moisture/water.
Click to expand...
The gas is brand new and I started with a clean/dry tank. There is no moisture/water anywhere in the system. Appreciate the suggestion though.
 
If you haven't done so already, take the drain plug out of the carb float bowl and open the shutofff valve to verify if "enough" fuel is getting delivered to the carb with everything buttoned up like it would be during operation.. A healthy dribble is the way I would describe the typical flow rate. It really doesn't take that much to run the engine.

You are correct about the plastic floats. All the carbs I've opened up in recent history have 100% plastic floats which renders float level essentially unadjustable.
 
A couple of things, check the float bowl vent. Is it covered up by the gasket between the carb and air filter housing? If you still have an old carb you can experiment with opening up the main jet a tiny bit. I have had success with a 0.1mm or .035 in drill bit held in a pair of pliers and spinning the jet with my fingers. Should be just a slight bit of material removed.
 
OK - here are some status updates:
  • If I pull the bowl, it will drain the tank in just a few minutes. IT IS NOT PLUGGED!!
  • Gas is brand new - gas can I transported the gas in was dry/empty when I filled it.
  • Retorqued head bolts per spec (19 ft-lb). They did not tighten any more than they were.
  • Zip-tied compression release mechanism out of the way - no change.
  • Found it has plenty of compression as I could barely turn it over by pulling the cord.
  • Bowl float vent - great idea and the new gaskets had the hole in the wrong place (slightly). I saw it over the weekend and fixed it prior to posting but forgot to mention it.
  • I removed the main jet entirely and it ran at full throttle - much richer than with the choke on and a jet installed (DUH!) If I turned on the choke it would shut down which is what I would expect.
  • Main jet measured close to 0.8mm in diameter. I tried 0.9 and saw very little difference. I drilled it to 1.0mm and I was able to turn off the choke more than I had prior but it still need it. Looks like I am onto something.
My father is in his mid 70's and does not always remember everything (all the time)... who does? I was talking to him today and he said he has had to have the choke on since the day he bought it (brand new) 10+ years ago for it to run properly. I am thinking tomorrow I am going to slowly increase the jet size until I can find a size that works. I have 3 of them to play in the event I make a mistake.

Is it best to start the mixture screw 2.5 turns out from seated??
 
Be very cautious about drilling jets. As little as a .005" increase can give very large increases in effective size depending on starting diameter.

Break out the calculator and tiny drill bit set - figure the current diameter, then using the formula for area of a circle (Pi x R squared) increase by 10 to 15 percent. Continue as necessary till she runs right.
 
I stopped drilling the main jet at 1.2mm and it will now run decently at only 1/2 choke. Full choke it is way too rich and it will surge and die. Anything less than half choke and the engine will surge but keep running. As the governer tries to maintain the proper RPM, it will spit fuel back out at me as it transitions. It has to be in the sweet spot which is pretty narrow. I will probably put the original jet back in that has the stock hole size. It was worth a try.

I checked the govenor and it was adjusted properly. If I disconnect it and control the throttle manually, I get the same surging so it is something going on with fueling.

More info on the origin of this log splitter. Apparently when it was purchased, it was "new" but purchased at an auction. I am guessing somebody bought it, couldn't get it running and then returned it back to the store where it eventually went up for auction. I bet this thing has had an issue from the day it was built... BUT WHY?? I am soooo frustrated. It should not be this difficult :-(
 
You seem to have thoroughly investigated and eliminated all the usual suspects for this type problem. Two new carbs should rule out a carb problem. Your other work would seem to rule out fuel delivery. It's hard to imagine how the choke would offset an ignition problem, but I would still waste $5 on a Harbor Freight spark tester just to prove ignition is solid. In light of what you have already done I don't think standard troubleshooting is going to gain you any more ground. It's time to start looking for the weird.

You have mentioned several symptoms that seem unusual and some even contradictory
1. The engine runs well at idle but not over 1/2 throttle,
2. It ran well for a couple tanks of gas and then went south. What changed?
3. It spits fuel back (and therefore you are getting reverse air flow) under certain conditions.
4. Choke or way oversized jets are required to deliver sufficient fuel. Is there an air leak somewhere, or is the reverse air pulse preventing the fuel from developing forward momentum through the jets.

If you can add any other symptoms to this list it you might be able to look at the list and say that there is some cause that explains all the symptoms. At this point if it was mine I would just take it apart and examine every piece for defects.. Check the valves and stems for bent, springs for force, timing on the cam, if compression release bleeds air through the valves examine that. At this point it becomes a research project.
 
FamilyMechanic said:
I stopped drilling the main jet at 1.2mm and it will now run decently at only 1/2 choke. Full choke it is way too rich and it will surge and die. Anything less than half choke and the engine will surge but keep running. As the governer tries to maintain the proper RPM, it will spit fuel back out at me as it transitions. It has to be in the sweet spot which is pretty narrow. I will probably put the original jet back in that has the stock hole size. It was worth a try.

I checked the govenor and it was adjusted properly. If I disconnect it and control the throttle manually, I get the same surging so it is something going on with fueling.

More info on the origin of this log splitter. Apparently when it was purchased, it was "new" but purchased at an auction. I am guessing somebody bought it, couldn't get it running and then returned it back to the store where it eventually went up for auction. I bet this thing has had an issue from the day it was built... BUT WHY?? I am soooo frustrated. It should not be this difficult :-(
Click to expand...
Are these problems under no load conditions or while splitting? If you run the engine up to max governed RPM, no load,(like you are ready to split a piece of wood) what will it do?
 
I had this problem on a Tecumseh snow king motor and it was the flywheel key about sheared off.
 
You must log in or register to reply here.

Similar threads

JHZR2
Old Briggs 3.5 Max governor spring
Replies
9
Views
535
Nukeman7
W
Which Choke Plate for B&S Replacement Carb
Replies
4
Views
2K
Whimsey
C
Flipping yard ope for profit?
2
Replies
33
Views
984
jeepman3071
garageman402
Generator Carb Problems
2
Replies
31
Views
2K
The_Eric
C
Stihl backpack blower quits after running fine for a couple of minutes?
Replies
19
Views
748
super20dan
Back
Top